CityJSON is a format for encoding a subset of the CityGML data model using JavaScript Object Notation (JSON).
A CityJSON file represents both the geometry and the semantics of the city features of a given area, eg buildings, roads, rivers, the vegetation, and the city furniture.

The aim of CityJSON is to offer an alternative to the GML encoding of CityGML, which can be verbose and complex (and thus rather frustrating to work with).
CityJSON aims at being easy-to-use, both for reading datasets, and for creating them.
It was designed with programmers in mind, so that tools and APIs supporting it can be quickly built.
It was also designed to be compact (we have noticed a compression factor of at least 4X, often 10X+), and friendly for web and mobile development.

A CityJSON object, representing a city, is as ‘flat’ as possible, ie the hierarchy of CityGML has been flattened out and only the city objects which are ‘leaves’ of this hierarchy are implemented.
This considerably simplifies the storage of a city model, and furthermore does not mean that information is lost.

A JSON encoding of GML, huh?!?

While its name otherwise implies, CityGML is not only a GML encoding, but is actually an open standardised data model to store digital 3D models of cities and landscapes.
It defines ways to describe most of the common 3D features found in cities, and the relationships between them.
It also defines different standard levels of detail (LoDs) for the 3D objects, which allows us to represent objects for different applications and purposes.

CityJSON can be considered as the third implementation of the CityGML data model.

Note

CityJSON is not an official standard, and it is not part of the OGC standards (unlike its parent standard CityGML).
It was started by the 3D geoinformation group at TU Delft (who is the main maintainer) and other developers are welcome to contribute.